1. Field of the Invention
The invention generally relates to gas concentrators, and more particularly relates to a concentrator having an integral system adapted for removing moisture from compressed gas feed stream to improve the performance efficiency of adsorbent beds. The application is particularly directed to compact; portable oxygen concentrators for therapeutic use, but the principles apply to any gas concentrator utilizing adsorbent beds.
2. Description of the Related Art
The application of oxygen concentrators for therapeutic use is known, and many variants of such devices exist. A particularly useful class of oxygen concentrators is designed to be portable, allowing users to move about and to travel for extended periods of time without the need to carry a supply of stored oxygen. Most of these portable concentrators are based on Pressure Swing Adsorption (PSA) or Vacuum Pressure Swing Adsorption (VPSA) designs which feed compressed air to selective adsorption beds. In a typical oxygen concentrator, the beds selectively adsorb nitrogen, resulting in pressurized, oxygen-rich product gas.
The main elements in a typical oxygen concentrator are shown in FIG. 1. Air is draw in, and typically filtered, at air inlet 1 before being pressurized by compressor 2. The pressurized air is directed by a valve arrangement through adsorbent beds 3. An exemplary adsorbent bed implementation, used in a concentrator design developed by the inventors, is three columns filled with zeolite powder. The pressurized air is directed through these columns in a series of steps which constitute a gas separation cycle, often a PSA cycle or some variation including vacuum instead of, or in conjunction with, compression. Although many different arrangements of beds are possible as well as a variety of different gas separation cycles, the result is that nitrogen is removed by the adsorbent material, and the resulting oxygen rich air is routed to a product gas storage device at 4. Some of the oxygen rich air can be routed back through the bed to flush out (purge) the trapped nitrogen to an exhaust 6. Generally multiple beds, or columns in the exemplary device, are used so at least one bed may be used to make product while at least one other is being purged, ensuring a continuous flow of product gas. The purged gas is exhausted from the concentrator at the exhaust 6.
Such systems are known in the art, and it is appreciated that the gas flow control through the compressor and the beds in a gas separation cycle is complex and requires precise timing and control of parameters such as pressure, flow rate, and temperature to attain the desired oxygen concentration in the product gas stream. Accordingly, most modern concentrators also have a programmable controller 5, typically a microprocessor, to monitor and control the details of the gas separation cycle and monitor various parameters. In particular, the controller controls the timing and operation of the various valves used to cycle the beds through feed and purge steps which make up the gas separation cycle. Also present in most portable concentrators is a conserver 7 which acts to ensure that oxygen rich gas is only delivered to a patient when a breath is inhaled. Thus less product gas is delivered than by means of a continuous flow arrangement, thereby allowing for smaller, lighter concentrator design. A pulse of oxygen rich air, called a bolus, is delivered in response to a detected breath via the conserver. A typical concentrator will also contain a user/data interface 8.
A portable oxygen concentrator must be small, light and quiet to be useful, while retaining the capacity to produce a flow of product gas, usually a flow rate prescribed by a medical practitioner, adequate to provide for a patient's needs. Although fixed site PSA based concentrators have been available for many years, such fixed site units may weigh 50 pounds or more, be several cubic feet in size, and produce sound levels greater than 50 dBA. A portable concentrator may weigh on the order of 10 lbs, be less than one half cubic foot in size, and produce as little as 35-45 dbA. The portable concentrators still need to produce the prescribed flow rate of oxygen to be considered beneficial for therapeutic use. Thus portable concentrators involve a significant amount of miniaturization, leading to smaller, more complex designs compared to stationary units. System size, weight and complexity may lead to fewer mitigative options or design choices against contamination and other wear and tear effects.
One particular challenge of portable concentrator design is that the adsorbent beds must by necessity be small, yet capable of producing an adequate quantity of product gas. Since the adsorbent beds are optimized for maximum performance from a reduced size, any significant decrease in capacity of the beds over time can result in decreased product purity. One contributing factor that can lead to a decrease in bed capacity is the adsorption of impurities that do not completely desorb during normal process operation, leading to the accumulation and retention of impurities in the beds. An example of such an impurity that reduces the adsorption capacity of many zeolites used in air separation is water. Some stationary concentrators utilize some means of removing water from the compressed gas before feeding the adsorbent beds. Portable concentrators, by the nature of their application, are more likely to be exposed to a wide range of operating conditions including high humidity environments and/or rapid temperature changes that could result in the need for more sophisticated water rejection capabilities than implemented in prior designs. If water is present, either in the form of liquid or vapor, and enters the molecular sieve beds, the beds will irreversibly adsorb at least some of this water during each adsorption cycle. The energy of adsorption of water on zeolites is very high and not all water adsorbed during the adsorption steps in the process is desorbed during evacuation/purge of the beds under typical cycling. Therefore, complete removal of adsorbed water from zeolite beds usually entails applying some sort of energy to the beds, such as thermal, infrared, or microwave, and purging with a dry gas or applying a vacuum to the beds during the regeneration process. These regeneration processes are impractical in a portable concentrator. As a result, the accumulation of adsorbed water over time results in a reduction in capacity of the beds, as fewer sites are available for nitrogen binding. Fewer binding sites in the adsorbent bed can result in a decrease in product purity over time, and ultimately a shortened service life of the concentrator. Many zeolites used in air separation, and in particular advanced adsorbents, particularly the high lithium containing low silica X type zeolite, are hydrophilic in their activated state and can therefore be prone to this problem. In the drive to make more compact and efficient devices, cycle frequencies increase, and adsorbent productivity increases accordingly with advances in process and adsorbent technology. The corresponding decrease in adsorbent inventory exacerbates the problem as the amount of gas processed per unit of adsorbent increases proportionally and the presence of impurities in the process gas can deactivate the adsorbents at a much faster rate than with conventional PSA processes, as described in U.S. Pat. Nos. 7,037,358 and 7,160,367, which are incorporated by reference herein.
It is therefore desirable to keep moisture out of the sieve beds without detrimentally affecting other characteristics of the portable oxygen concentrator. Although highly effective air drying systems exist in other fields, most of these systems consume power, increase size and weight, or reduce system efficiency in a manner detrimental to the stringent power consumption, size/weight, and acoustic noise level requirements of portable concentrators. Using a single process bed with some portion of the bed dedicated to impurity processing/rejection is a common method of adding impurity rejection to a gas separation system. Adding beds dedicated to dehydration of the feed stream upstream of the zeolite beds or implementing layered adsorbent beds utilizing desiccants in addition to adsorbents suited for the desired gas fractionation are also common methods of adding water rejection capacity to a gas separation system, and can be effective in many circumstances. However, additional beds add significant size and weight to the concentrator, or in the case of layered beds the desiccant layer displaces volume that could otherwise be used for adsorbent used for highly efficient air separation or the volume of the process columns could be decreased accordingly, and additional power is used to compress gas through this desiccant. The desiccants typically used for pre-drying air are also prone to deactivation during constant cycling as well as during shutdown periods, and are often regenerated via applying one of the aforementioned methods. In some cases, the desiccant layer may be advantageous, but also might not be entirely effective at protecting the specialized adsorbents from water damage. By their nature, personal oxygen concentrators, be they portable or stationary, often operate in varied usage modalities rather than in the continuous duty manner of an industrial gas production plant. The duty cycle, storage time between use, and storage environment, can vary widely from unit to unit. For example, home health care providers may have a fleet of units that are stored in warehouses that are not climate controlled while waiting for delivery to patients for use. Similarly, patients may store units in their car or home for a given period of time without use depending on their individual oxygen needs. U.S. Pat. Nos. 7,037,358 and 7,160,367 teach that extreme care must be used in shutting down and storing PSA units that are run on an intermittent basis. Any water (or other impurities) remaining in the desiccant layer(s) or portion of the bed used for feed gas drying upon shutdown will diffuse over time due to the gradient in chemical potential between the portion of the bed that is used for impurity removal during normal operation and the dry portion of the beds. The diffusion coefficient of water in zeolites has an Ahrennius type temperature dependence, so if a concentrator is stored in a high temperature environment the rate of intraparticle diffusion will increase exponentially with temperature. The gas phase diffusion rate will increase with increasing temperature as well. The above referenced patents disclose many preventative measures that can be performed during shutdown and storage of PSA units that are operated intermittently to mitigate these issues. In an oxygen concentrator it is advantageous to remove as much water as possible from the compressed gas feed stream to prevent deactivation of the highly efficient zeolite, use less desiccant, and minimize the presence of water in the beds during shutdown. Traditional means of removing water such as coalescing filters and gravity water traps have limited abilities to remove water and can thereby limit the usable service life of oxygen concentrating equipment. The varying operating and storage environments that portable concentrators may be exposed to result in design challenges that more conventional gas separation systems such as gas separation plants might not encounter and must be addressed.